It's not just having a "plan", actual software development needs to start to begin pretty soon here. A 3 year dev cycle factoring in some bumps along the way, you need to have actual game development underway like this summer/fall and even that may be cutting it tight because Switch to Switch 2 is probably a generational leap whereas Wii U to Switch was basically the same tier hardware just a bit better in some respects. So you have to account developers needing a little more time to take full advantage of much better hardware than they had before.
So for us as consumers while Switch 2 may be years away, for Nintendo it really needs to be top of mind like now. As a plan yes, but even as actual game development and staff being allocated to these projects.
What does any of this have to do with what I said though? I don't disagree, I just said replacing Switch in 2021 would be foolishly premature.
Not sure where you're getting "replace the Switch in 2021" from, don't think anyone even the OP has said that. A phased transition is something different. While I think there is some merit to it, Nintendo is prone to not being able to do even standard transitions very well, that they probably shouldn't try to get too cute.
Plan for March 2023 Switch 2 launch and support the Switch through 2024, basically same as you did with the Switch and 3DS. Getting too fancy or cute might be doable but when you have so much trouble with transitionary periods, probably just better to keep it simple.